The present invention relates to an improvement in a steering reaction force (i.e., force fedback to a driver from a power steering device) control apparatus of a power steering device, for obtaining a proper steering reaction force in accordance with various running conditions of a vehicle such as a running speed of vehicle (to be referred to as a vehicle speed hereinafter) and a steering angle.
A power steering device has been widely used in various vehicles such as small-sized cars. Such a power steering device reduces a steering reaction force of a driver by its auxiliary steering output force (i.e., power assist force) and therefore can achieve many effects, e.g., can allow a light steering operation and reduce fatigue of the driver. Various arrangements for the power steering device have been conventionally known.
In a power steering device of this type, an operation of the device must be properly controlled in accordance with a steering load or a steering angle corresponding to a steering operation of a driver and various running conditions of a vehicle such as a vehicle speed, thereby obtaining a required auxiliary steering output force. That is, in a vehicle or the like which incorporates a power steering device of this type, a large auxiliary steering output force must be output to obtain an extremely small steering reaction force during a steering operation performed when the vehicle is parked or running at a low speed. However, when the vehicle is running at a high speed, such a large auxiliary steering output force generated when the vehicle is running at a low speed unnecessarily reduces an operation force of a steering wheel. Then, the driver feels uneasy, and hence this is not preferable to allow safe and comfortable driving. As a result, the driver will feel uneasy steering the automobile. Hence, such consequence is not preferable for safe and comfortable driving. Similarly, such a steering reaction force must be controlled to be increased as a steering angle is increased.
For this purpose, steering reaction force control apparatuses which utilize a hydraulic reaction force and can control rigidity (steering reaction force) of a steering wheel when a vehicle is running at a high or low speed have been conventionally adopted. A large number of apparatuses of this type having various arrangements have been proposed. For example, Japanese Patent Laid-Open No. 61-155059 discloses a power steering device (to be sometimes referred to as a PS hereinafter) shown in FIG. 12. In FIG. 12, reference numeral 10 denotes a PS oil pump (to be referred to as a main pump hereinafter) which is driven by an engine 102 of an automobile. The main pump 100 supplies an operation oil as pressure oil contained in an oil tank 103 to a power cylinder 104. Note that in FIG. 12, reference numerals 105a and 105b denote a PS pressure supply path for supplying the pressure oil from the main pump 100 to the power cylinder 104 and a returning path for returning it to the tank 103. Reference numeral 106 denotes a PS main body having the power cylinder 104 and a rotary flow path switching valve. The PS main body 106 constitutes a portion of a steering link mechanism 108 disposed between left and right wheels 107 and is connected through a steering shaft 109a to a steering whheel 109 which is operated in a desired direction.
As is well known, the PS main body 106 has an arrangement as shown in FIGS. 13 and 14. In FIGS. 13 and 14, reference numeral 110 denotes a pinion shaft provided at the valve output side and having a pinion 110a which meshes with a rack 111 constituting the steering link mechanism 108, and reference numeral 112 denotes an input shaft (steering shaft 109a) which is coaxially connected to the right end of the pinion shaft 110 through a torsion bar 113 and the right end of which is connected to the steering wheel 109. Each of the shafts 110 and 112 is rotated in its steering direction. In body 114 which constitutes the PS main body 106, a rotor 116 (which is formed integrally with the shaft 112) and a sleeve 117 for constituting a rotary flow path switching valve 115 are provided integrally with the shafts 112 and 110 so that flow paths between left and right cylinder chambers (C1 and C2) of the power cylinder 104 and the main pump 100 and the tank 103 are switched by relative rotational displacement between the rotor 116 and sleeve 117. It is a matter of course that the rotor 116 at the input side is connected integrally with the sleeve 117 at the output side through a fail-safe mechanism so that each member can be pivoted through a predetermined angle. Note that since an arrangement and an operation of such rotary flow path switching valve 115 are conventionally well known, a detailed description thereof will be omitted.
Reference numeral 120 denotes a hydraulic reaction force chamber which is provided between the input side and output side members (112 and 110) constituting the flow path switching valve 115, and which constitutes a steering force control apparatus for restricting a portion between the members 112 and 110 by a biasing force of at least a pair of plungers 121, thereby restricting rotation of an input side member (in this case, an arm portion 112a). The hydraulic reaction force chamber 120 is arranged such that the pressure oil is supplied from an oil pump 201 for supplying the pressure oil is properly controlled to be supplied to the hydraulic reaction force chamber 120. In FIG. 14, reference numeral 122 denotes a small fixed orifice for returning reaction force hydraulic pressure supplied to the hydraulic reaction force chamber 120 to the tank 103. The fixed orifice 122 is arranged such that the reaction force hydraulic pressure flowed into the hydraulic reaction chamber 120 is flowed to a tank side chamber 123 at a small flow rate.
When the reaction force oil pressure is supplied onto the hydraulic reaction force chamber 120, relative rotation between the input side and output side members (112 and 110) is restricted in accordance with the magnitude of the oil pressure to generate a proper steering reaction force, thereby performing predetermined steering reaction force control.
A control valve 130 for supplying the reaction oil pressure to the hydraulic reaction force chamber 120 which controls the steering reaction force as described above is arranged such that it is driven by a solenoid 134 which is controlled by the controller 133 which receives at least vehicle speed information and steering angle information as vehicle running condition information from a vehicle speed sensor 131 and a steering angle sensor 132, respectively, thereby obtaining desired characteristics.
Note that reference numeral 135 denotes a reaction force oil pressure supply side path.
However, in such a conventional apparatus, a seal 120a is used to prevent oil leakage in the hydraulic reaction force chamber 120 and therefore friction is produced at this portion. This frictional force is not a problem on a road under normal conditions. However, since a reaction force from the wheels is reduced on a road having a low road surface resistance (low .mu. road) such as a road covered with snow, the above frictional force becomes a value which cannot be neglected with respect to the steering reaction force. As a result, it becomes difficult to transmit information of displacement and force from the wheels to a driver, thereby interfering with a smooth steering operation.